Vehicles with pneumatic brakes normally have spring-operated parking brakes which cannot be released until a certain minimum air pressure is available in the vehicle's air-pressure reservoir. The spring-operated parking brakes can be set by releasing air pressure from them even when adequate reservoir air pressure exists.
Pressure holding valves are known in the art which hold maximum reservoir air pressure in the spring-operated parking brakes even after the air pressure has been decreased to some intermediate value. These pressure holding valves begin releasing air or pressure at some low value of air. pressure. The point at which prior art pressure holding valves begin releasing air is unpredictable.
Wheel slip control systems, for example, of the type described in U.S. Pat. No. 3,951,467 and assigned to the assignee of the present invention add an additional complication to the performance of the pressure holding valve. The wheel slip control system periodically releases brake pressure when certain measured conditions indicate that a skid is occurring. If not prevented from doing so, the driver could intentionally or accidentally, defeat the operation of the wheel slip control system by operating the parking brake valve while also braking with the normal control. This could lock the vehicle wheels and cause a potentially dangerous skid.